A retort is often utilized to provide a controlled atmosphere during heat treating. The retort used within a furnace has a gas inlet and outlet. Gas leakage can be eliminated by sealing the retort with a lid. In this system, retort exhaust never comes in contact with the furnace; the gas outlet port is connected to a ventilation source. However, limitations arise due to the difficulty in securing and removing the lid.
A sand sealed retort, commonly known in the art, employs the basic design of the sealed lid retort, with sand used to seal the lid which is either clamped on or held on by its own weight; allowing easy lid removal. The gas enters the retort through an inlet port and exits through the sand seal, due to a pressure gradient which also prevents furnace atmosphere infiltration into the retort. This process allows the exhaust gas to contact the inner surface of the furnace; an acceptable occurrence only if either the gases are inert, or furnace damage is acceptable.
Retort systems which employ a sand seal are used for bonding carbon substrates, diffusion coating, and annealing. (U.S. Pat. Nos. 4,241,104, 4,293,338, 4,310,302, and 4,504,957, incorporated herein by reference).
A similar technique for maintaining the desired retort environment is disclosed in U.S. Pat. No. 4,701,127 (incorporated herein by reference). A deformable metallic foil gasket which is sufficiently porous to permit outflow of purging gas is utilized instead of a sand seal.
Although the use of a sand seal or deformable metallic foil gasket allows for easy lid removal, there are inherent disadvantages. Furnaces can seldom be completely sealed from leakage into the atmosphere. Since the gases flow from the retort into the furnace, the escape of hazardous gases into the atmosphere becomes a real problem. Furthermore, even if the gases are not "hazardous", they still may contaminate or damage the furnace itself, adding the cost of cleaning or replacing the furnace. Improved techniques which impede the escape of gases and prevent the contamination or damage of the furnace, while still allowing for easy lid removal, are sought.